The present application claims priority under 35 U.S.C. xc2xa7 119 based upon Swiss Application No. 2000 1308/00, filed on Jul. 3, 2000.
The present invention relates to mounting of semiconductor chips. More particularly, the present invention relates to a method and an apparatus for mounting semiconductor chips onto a flexible substrate.
In the semiconductor industry, apart from the proven metallic substrates, flexible substrates such as BGA(copyright) flextapes are increasingly being used. Furthermore, such flextapes are becoming increasingly thin and reach thicknesses of only 50 xcexcm. This leads to difficulties with the mounting of semiconductor chips which are manifested by irregular thickness of the adhesive between the semiconductor chip and the flextape, inclined (tilt) and incorrect position of the semiconductor chip. In addition, it can happen that, before finally hardening, the adhesive flows about, contracts, separates locally in various phases, etc., which can strongly impair the quality of the adhesive bond. Contamination of the bond pads can also result from this.
The object of the invention is to propose a method and a device with which flexible substrates can be equipped with semiconductor chips in reliable and high quality.
Solving of the task is achieved in accordance with an embodiment of the invention in that the substrate is held flat on a level support surface at least during hardening of the adhesive preferably however during all critical phases of the mounting process.
With a first solution, the automatic assembly machine, a so-called die bonder, is equipped with a heating plate in order to harden the adhesive immediately after applying the semiconductor chip. The heating plate has channels or grooves on the support surface facing the substrate to which vacuum can be applied in order to hold the substrate flat until the adhesive has hardened. In addition, the automatic assembly machine is preferably equipped with a support plate which also has channels or grooves to which vacuum can be applied in order to suck the entire area of the substrate onto a level support surface during application of the adhesive and during placement of the semiconductor chip.
This solution is particularly suitable for the processing of so-called matrix substrates with which the substrate sites for the semiconductor chips are arranged next to each other on the substrate in blocks of n rows and m columns. Mounting of the semiconductor chips takes place as usual in three steps: In the first step, adhesive is applied to the substrate sites at a dispensing station. In the second step, the semiconductor chips are placed onto the substrate sites at a bonding station. In the third step, the adhesive is hardened.
In order that sufficient time is available for the hardening of the adhesive, processing of such a matrix substrate takes place in blocks as follows: As soon as a block with its nxc3x97m substrate sites is completely equipped with semiconductor chips, the vacuum is released and the substrate is advanced in transport direction. At the same time, the support plate and heating plate are moved back in the opposite direction to the transport direction to a predetermined start position. Afterwards, vacuum is applied to the channels of the support plate as well as the heating plate so that the substrate lies flat and is fixed on the support plate and also on the heating plate. Subsequently, adhesive is applied to the next nxc3x97m substrate sites at the dispensing station and, at the bonding station, the substrate sites to which adhesive has already been applied are equipped column for column with semiconductor chips. When a column is fully equipped, then, without releasing the vacuum, the support plate and the heating plate are advanced together in transport direction for equipping the next column. The substrate therefore remains fixed on the support plate and the heating plate and is moved with them. After equipping the last column of a block, the next cycle begins in the way described by releasing the vacuum, advancing the substrate and returning the support plate and the heating plate.
Doubling or triplicating of the curing time can be achieved in that the heating plate is formed with two or three heating positions. In order that the throughput rate of the automatic assembly machine can be maintained relatively high, relatively fast setting adhesives must be used with this solution.
With certain applications, it suffices when the adhesive is only partially cured, eg., 80%. The remaining hardening then takes place in a subsequent process step, for example, during wiring on a wire bonder.
With a second solution, the hardening of the adhesive does not take place on the automatic assembly machine, but in an oven. During the time spent in the oven, it is ensured that the substrate lies flat on a support. A plate with channels or grooves to which vacuum can be applied so that the substrate can be sucked on to it serves as the support. The plate itself can, but must not necessarily, be heated. With this solution, slowly setting adhesives can also be used.
The invention results in a great improvement in the mounting quality. Because the substrate is held flat during hardening of the adhesive, an adhesive layer of constant thickness is formed between the substrate and the semiconductor chip: During hardening, the adhesive can no longer bend the substrate. Furthermore, a greater homogeneity of the adhesive is achieved while the customary hardening, the different components of the adhesive can segregate, for example, into areas of a lot of silver and areas with little silver, or even voids can occur.
Furthermore, the lying flat of the substrate on the heating plate guarantees the reliable heat transfer from the heating plate to the adhesive. This is of great importance as the quality of the heat transfer has a decisive influence on the curing time necessary for the complete hardening of the adhesive.
In the following, embodiments of the invention are explained in more detail based on the drawing.